- Email: [email protected]
Characteristic changes under pulsed pressure action in electrode materials based on LiMn2O4 and Li4Ti5O12 spinels
Solid State Ionics 177 (2006) 2779 – 2785 www.elsevier.com/locate/ssi
Characteristic changes under pulsed pressure action in electrode materials based on LiMn2O4 and Li4Ti5O12 spinels A.V. Nikonov a,⁎, E.M. Kelder b , J. Schoonman b , V.V. Ivanov a , N.M. Pivkin c a
b
Institute of Electrophysics UD RAS, Ekaterinburg, Russia Delft University of Technology, Department of DelftChemTech, The Netherlands c Institute of Polymeric Materials, Perm, Russia
Received 4 October 2005; received in revised form 23 June 2006; accepted 4 July 2006
Abstract Magnetic pulsed compaction (MPC) method has been applied to compact the electrode materials for lithium rechargeable batteries based on LiMn2O4 and Li4Ti5O12 spinels (LMO and LTO). The compressibility and structure evolution of these materials under action of pulsed pressures up to 0.7 GPa have been studied. It was found that pulsed compaction decreases the adhesion strength of electrode coatings with Al foil, however following heat treatment at temperatures up to 220 °C leads to partial restoration of adhesion strength. The pulsed pressing of the LMO electrodes does not noticeably influence the electric characteristics of the cells based on them, but cycleability of the LTO electrode cells improves greatly after the compaction treatment. At the same time, the temperature treatment does not essentially influence the operation of both types of cells. © 2006 Elsevier B.V. All rights reserved. Keywords: Pulsed compaction; Electrode material; Compressibility; Adhesion strength; Lithium rechargeable batteries
1. Introduction At present time considerable efforts are applied for the development of new electric energy sources and improvement of ones that are already known. In particular, great attention is focused on rechargeable Li-ion batteries [1,2]. In order to improve their characteristics the researchers mainly try to find new and more perfect electrode materials (for example, see [3–5]). Recently [6], another approach for improvement of the batteries operation by usage of pulsed pressing has been suggested. It has been shown that dynamic compaction substantially decreases the internal resistance of all-solid-state Li-ion batteries. Therefore the influence of pulsed pressure on electrodes characteristics is interesting to investigate in details. Earlier the authors developed the magnetic pulsed compaction (MPC) [7] as the method for the production of ⁎ Corresponding author. E-mail address: [email protected] (A.V. Nikonov). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.07.001
new types of bulk nanostructured materials for various structural and functional purposes. In this type of pressing, a soft pulsed compression wave up to 2 GPa in amplitude propagates in the powder for 10–1000 ms, so that the velocities of material displacement reach 10–100 m/s. These conditions, in combination with adiabatic heating, are favorable for effective overcoming of the strong particles coupling in nanopowders [8–10]. This provides compacted powder states of increased density, promotes destruction of particle agglomerates, and favors structural transformations resulting in microdistortions of the lattice, defects, and phase transitions. Thus the MPC seems to be a suitable method for pressure treatment of electrode coatings. Li-containing spinels are considered to be the promising candidates as electrode material [11]. Therefore in this work the influence of pulsed compaction treatment on characteristics of two types of well studied materials—LiMn2O4, Li4Ti5O12 [11–13] has been investigated. Compressibility, structure, adhesion strength and electrochemical characteristics of these electrode materials have been studied.
2780
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
2. Experiment Two types of electrodes based on the commercial nanopowders LiMn2O4 and Li4Ti5O12 (Honeywell) have been used in our experiments. The powders were mixed with graphite, carbon black and polyvinyl difluoride (PVdF) binder at ratio of 80:10:3:7 by weight. Then the solvent (NMP) was added in the mixture until the slurry was obtained. The slurry was spread on aluminium foil used as a substrate and then dried at the temperature of 140 °C during 1 h under vacuum. The tailored electrode coatings based on LiMn2O4 and Li4Ti5O12 powders are abbreviated below as LMO and LTO, accordingly. The compressibility of both electrode materials has been investigated using uniaxial MPC treatment. The pressure pulses were characterized by amplitudes of up to 0.7 GPa and of about 300 μs in duration. The disk-shaped samples 32 mm in diameter have been compacted between two polished steel disks. In order to investigate the influence of heat treatment on the compacts properties a series of samples was pressed at the same pressure of 0.45 GPa and then annealed at different temperatures up to 220 °C during 3 h. The densities of the coating materials were determined via the thickness measurements with standard micrometer before and after compaction. The phase composition and structure of the materials were analyzed by X-ray diffraction (XRD) method using a DRON-4 diffractometer with filtered CuKα radiation. The microstructure of electrode materials was observed by atomic force microscopy (device Solver 47p) in profile (HEIGHT) and phase contrast (MAC⁎Cos) modes. The tensile adhesion strength between electrode material and aluminium substrate was measured on disk-shaped samples 22 mm in diameter. In order to perform a test, a sample was glued between flat surfaces of two metal parts of the tear testing machine. Then these two parts were pulled in different directions until the coating was detached from the substrate. This process was completely force controlled. The preliminary experiments had shown that the strength of gluing layer was 20 ± 0.5 MPa allowing to carry out the measurements of mechanical characteristics for the coatings with σ < 20 MPa. In order to investigate the influence of MPC and temperature treatments on electrochemical characteristics of the electrode
Fig. 2. X-ray diffraction patterns of uncompacted and compacted LMO and LTO coatings. C—graphite, A—aluminium.
materials, the comparison of operation of coin-cells made with the usage of electrodes prepared by different regimes has been made. Thereat the tested electrodes were fabricated from both compacted and uncompacted electrode foils. The pure Li was used as the counter electrode. The electrolyte was a liquid (1 M LiPF6 in EC:DMC 1:1) dispersed in a polymer separator material (Solupor). Both foil electrodes were used to assemble the cylindrical shape cells which are perspective for the commercial production. The pile which consisted of the anode (LTO) foil, the separator (Solupor), the cathode (LMO) foil and the insulator (Kapton) foil was coiled and placed into aluminum tube. The electrolyte was the same solution as used in the coin-cells. Two types of so prepared cylindrical cells have been tested. The first one was radially compacted by the use of IAP Magnepress, and the second one was not compacted. All cells were charged and discharged at a constant current rate and cycled within a certain voltage range. All the measurements were controlled and recorded automatically by a MACCOR S4000 battery tester. 3. Results and discussion
Fig. 1. Compressibility of LMO and LTO coating under action of pulsed pressure.
The density data of compacted electrode coatings are presented in Fig. 1. The symbols denote values of experimental data averaged by 10 samples and solid lines are their spline approximations. The densities of the electrodes coatings are shown in relative units for better comparison of the materials compressibilities because they have different theoretical densities. The relative density γ is the relation of the measured coating density to the theoretical density of the same coating material. The theoretical densities of LMO and LTO coatings, that were calculated based on the densities of the material components and their proportion in the composition, were
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
3.54 g/cm3 and 3.02 g/cm3, accordingly. Maximum densities of LiMn2O4 and Li4Ti5O12 have been found by X-ray investigation and were 4.28 g/cm3 and 3.51 g/cm3, accordingly. PVDF density was 1.78 g/cm3 and the density of carbon black was equal to the density of graphite and was 2.27 g/cm3. It can be seen that compressibilities of the coatings are practically equal though experimental data for LMO coatings stably exceed LTO data. The highest densities of LMO and LTO coatings compacted with pressure ∼ 0.7 GPa were 96% and 91%, accordingly. The X-ray research showed (see Fig. 2), that in both electrode materials, LMO and LTO, there were no substantial structural changes upon pressing and annealing. Some graphite and metallic aluminum diffraction lines can be seen at the X-ray diagrams of compacted coatings along with the base phase
2781
diffraction lines of active material. The relative intensities of LiMn2O4 and Li4Ti5O12 diffraction lines of pressed coatings are similar to those observed in the starting materials within the error range for their determination. The analysis of integral diffraction line widths showed that at small angles they are almost the same, but the line broadening in the large angles area was observed. This effect was more severe as higher pulsed pressure was applied. This line broadening can not to be explained by the decreasing of the particle size. Basic reason of the line broadening is the distortion of the spinel crystalline lattice both due to microdistortions and to packing defects. Figs. 3 and 4 illustrate the comparison of AFM images of starting, compacted and additionally annealed LMO and LTO coatings, respectively, in phase contrast mode. For both starting
Fig. 3. AFM images (MAG⁎cos mode) of the external surface of LMO coatings: a) uncompacted; b) compacted (0.45 GPa); c) compacted (0.45 GPa) and annealed (180 °C); d) compacted (0.45 GPa) and annealed (220 °C).
2782
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
Fig. 4. AFM images (MAG⁎cos mode) of the external surface of LTO coatings: a) uncompacted; b) compacted (0.45 GPa); c) compacted (0.45 GPa) and annealed (180 °C); d) compacted (0.45 GPa) and annealed (220 °C).
materials the images (Figs. 3a and 4a) reveal the structures with mean particle size in the range of 200–1000 nm. From these images, one phase seems to be dominant, which is probably associated with polymer-covered particles of active materials. The images of compacted coatings (Figs. 3b and 4b) obtained at pulsed pressure of 0.45 GPa show very tight packing of crystals that corresponds to almost full dense material. The fine-grained structure with mean size well below 100 nm is seen. It is clear that the destruction of agglomerates during MPC treatment takes place. Probably, the starting active powders of LMO and LTO consist of sub-micron agglomerates, which are covered by a polymer in the starting coatings. The next feature is the presence of at least two phases on the surfaces of compacted coatings. Light and dark zones on these images denote various phases. Additional annealing of the compacted coating results in more homogeneous distribution of polymer additive (Figs. 3c
and 4c). Moreover the annealing at the highest temperature (220 °C) led to polymer removal from the external surfaces (domination of light zones on Figs. 3d and 4d). Materials that were treated under such regime are characterized with the finest surface structure. In our opinion, the polymer transforms to the liquid state and penetrates inside the pores of the coating. Pulsed compaction decreases the adhesion strength of LMO and LTO coatings with Al foil as it is demonstrated in Fig. 5. The adhesion strength decreases upon increasing of the compacting pressure. But the annealing of the coatings at temperatures up to 220 °C after pulsed compaction leads to the partial restoration of adhesion strength in comparison with the strength that was measured before compaction (Fig. 6). In the case of LMO this value was up to 8,4 MPa, and for LTO coatings—11 MPa that was close to the adhesion strength of the starting coatings.
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
2783
Fig. 5. Influence of compacting pressure on adhesion strength for LMO and LTO coatings.
Most likely the reason of the adhesion strength decreasing during compaction is the destruction of the polymer layer binding of the coatings with the substrate due to pressing of the active material particles into it. The restoration of the adhesion strength after annealing is connected with the polymer binder viscosity decreasing upon heating, which leads to its flow and reconnection of the powder particles with the substrate. Fig. 7 shows the most typical experimental cycling results. It is clearly seen that MPC treatment of LMO electrode does not result in a considerable change in capacity behavior, while in the case of LTO electrode, cycleability (slope angle of the curve) and stability (absence of peaks on the curve) of cell operation has been improved after pulsed compaction essentially. A dependence of the cell capacity from the annealing temperature was not found for both electrode materials. Thereat the cells capacity with electrodes that were treated under different regimes fluctuated within 7% and 13% in the case of LTO and LMO, accordingly.
Fig. 6. Influence of annealing temperature on adhesion strength for uncompacted and compacted (0.45 GPa) LMO and LTO coatings.
Fig. 7. Capacity vs. cycle number for cells with uncompacted and compacted tested electrodes.
In Fig. 8 typical curves depicting behavior of the cells in the “charge–discharge” cycle are presented. These data also demonstrate no influence of pressure treatment on LMO electrode cells operation, while the cells that were made on the basis of compacted LTO electrodes showed more stable operation in comparison with uncompacted ones. It can be seen
Fig. 8. Typical charge–discharge curves of the uncompacted and compacted (0.45 GPa) LMO (a) and LTO (b) electrode cells.
2784
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
that gaps between “charge” and “discharge” shoulders greatly decrease after the electrodes were compacted. It shows qualitatively the decrease of the internal energy loss in the coin-cell as a result of the compaction. The numerical evaluation of internal energy loss for the 10th “charge–discharge” cycle has been calculated for the tested cells by the following formula: d¼
W0 −W 100 W0
where δ—loss factor, W0—energy introduced in the cell during the charge, W—energy returned by the cell during the discharge. In Fig. 9 the portion of the lost energy per cycle versus the annealing temperature is presented. In the case of the LMO electrodes the loss factor does not depend on a heat treatment and is approximately equal to 3%. The loss factor of the compacted LTO electrode cells is ∼ 7% that is much lower than the loss level, 22%, for the uncompacted ones. In all cases the temperature dependence of the loss factor is negligible. As it was shown above significant changes of the chemical composition and structure of the coatings under action of pulsed compaction did not take place. Therefore most likely the reason of the enhancement of electric characteristic of the coin-cells with LTO electrodes is the change of the electrical contacts between particles inside the coatings and also the improvement of contact between the active material and the substrate. As it can be seen from Fig. 3 and 4 Li4Ti5O12 powder is more agglomerated in comparison with LiMn2O4 powder, therefore it has less contact area. After compaction the particles size sufficiently decreased resulting in contact improvement. The decreasing of LiMn2O4 particle size is insignificant that is probably explained by the indifference of the LMO coating to pressure treatment. The pressure treatment reduced the adhesion strength between the electrode coating and the substrate. In experimental conditions it did not influence the electrical characteristics of cells. Nevertheless the given parameter is important for a long-term service of lithium batteries, and as was shown above, can be restored by the subsequent heat treatment.
Fig. 10. Capacity vs. cycle number for uncompacted and compacted cylindrical cells.
In Fig. 10 the cycleability tests for cylindrical cells assembled with the LMO electrode as cathode and LTO electrode as anode data are presented. It can be seen that the compacted cell is characterized by the higher starting capacity and much lower falling rate of capacity in comparison with uncompacted cell. It is possible to explain the improvement of the electrical characteristics of the cylindrical cells processed by radial pulsed pressure after assembly, both by the improvement of structure of the electrodes, and by the reduction of interelectrode gaps. 4. Conclusions 1. The electrode coatings based on LiMn2O4 (LMO) and Li4Ti5O12 (LTO) are characterized by good compressibility under pulsed compaction. The relative densities reached 96% and 91%, accordingly, upon pressure pulses with the amplitude of ∼ 0.7 GPa. 2. Under the pulsed compaction the agglomerates (0.2–1 mkm) of active material break into the particles with the size well below 100 nm. There were no essential changes in crystalline structure (XRD) of the electrode coating under action of pulsed pressures up to 0.7 GPa. 3. Pulsed compaction decreased the adhesion strength of LMO and LTO coatings with Al foil. Moreover the adhesion strength decreased with the increase of the compacting pressure. However the annealing of the samples at temperatures of up to 220 °C after pulsed compaction led to the partial restoration of adhesion strength. 4. Preliminary pulsed pressure treatment of LTO electrode improved the cycleability and stability of cells operation essentially. The same treatment did not influence the operation of LMO electrode cells. 5. The electrical characteristics of cylindrical cells have been significantly improved by radial pulsed pressure treatment. References
Fig. 9. Loss factor of the uncompacted and compacted (0.45 GPa) LMO and LTO electrode cells for 10th cycle.
[1] A.N. Jansen, A.J. Kahaian, K.D. Kepler, P.A. Nelson, K. Amine, D.W. Dees, D.R. Vissers, M.M. Thackeray, Journal of Power Sources 81–82 (1999) 902.
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785 [2] D. Aurbach, Journal of Power Sources 146 (2005) 71. [3] A. Odani, A. Nimberger, B. Markovsky, E. Sominski, E. Levi, V.G. Kumar, M. Motiei, A. Gedanken, P. Dan, D. Aurbach, Journal of Power Sources 119–121 (2003) 517. [4] J.T. Son, Electrochemistry Communications 6 (2004) 990. [5] J. Molenda, Solid State Ionics 176 (2005) 1687. [6] M.J.G. Jak, F.G.B. Ooms, E.M. Kelder, W.J. Legerstee, J. Schoonman, A. Weisenburger, Journal of Power Sources 80 (1999) 83. [7] V.A. Mironov, Magnetic Pulsed Compaction of Powders, Znanie, Moscow, 1980. [8] V.V. Ivanov, S.N. Paranin, A.N. Vihrev, A.A. Nozdrin, Materials Sciences Transactions 5 (1997) 49.
2785
[9] V.V. Ivanov, A.A. Nozdrin, S.N. Paranin, S.V. Zayats, Physics–Chemistry of Ultradispersed Systems, 2001, p. 229. [10] V.V. Ivanov, S.N. Paranin, S.V. Dobrov, A.V. Nikonov, V.R. Khrustov, Proceedings of Ninth Int. Conf. Megagauss Magnetic Field Generation and Related Topics, Moscow, St.-Petersburg, July 7–14 2002, p. 132. [11] M.M. Thackeray, Journal of the American Ceramic Society 82 (1999) 3347. [12] E.M. Kelder, F.J.B. Ooms, R. Perego, J. Schoonman, Journal of Power Sources 68 (1997) 590. [13] K.M. Colobow, J.R. Dahn, R.R. Haering, Journal of Power Sources 26 (1989) 397.
Characteristic changes under pulsed pressure action in electrode materials based on LiMn2O4 and Li4Ti5O12 spinels A.V. Nikonov a,⁎, E.M. Kelder b , J. Schoonman b , V.V. Ivanov a , N.M. Pivkin c a
b
Institute of Electrophysics UD RAS, Ekaterinburg, Russia Delft University of Technology, Department of DelftChemTech, The Netherlands c Institute of Polymeric Materials, Perm, Russia
Received 4 October 2005; received in revised form 23 June 2006; accepted 4 July 2006
Abstract Magnetic pulsed compaction (MPC) method has been applied to compact the electrode materials for lithium rechargeable batteries based on LiMn2O4 and Li4Ti5O12 spinels (LMO and LTO). The compressibility and structure evolution of these materials under action of pulsed pressures up to 0.7 GPa have been studied. It was found that pulsed compaction decreases the adhesion strength of electrode coatings with Al foil, however following heat treatment at temperatures up to 220 °C leads to partial restoration of adhesion strength. The pulsed pressing of the LMO electrodes does not noticeably influence the electric characteristics of the cells based on them, but cycleability of the LTO electrode cells improves greatly after the compaction treatment. At the same time, the temperature treatment does not essentially influence the operation of both types of cells. © 2006 Elsevier B.V. All rights reserved. Keywords: Pulsed compaction; Electrode material; Compressibility; Adhesion strength; Lithium rechargeable batteries
1. Introduction At present time considerable efforts are applied for the development of new electric energy sources and improvement of ones that are already known. In particular, great attention is focused on rechargeable Li-ion batteries [1,2]. In order to improve their characteristics the researchers mainly try to find new and more perfect electrode materials (for example, see [3–5]). Recently [6], another approach for improvement of the batteries operation by usage of pulsed pressing has been suggested. It has been shown that dynamic compaction substantially decreases the internal resistance of all-solid-state Li-ion batteries. Therefore the influence of pulsed pressure on electrodes characteristics is interesting to investigate in details. Earlier the authors developed the magnetic pulsed compaction (MPC) [7] as the method for the production of ⁎ Corresponding author. E-mail address: [email protected] (A.V. Nikonov). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.07.001
new types of bulk nanostructured materials for various structural and functional purposes. In this type of pressing, a soft pulsed compression wave up to 2 GPa in amplitude propagates in the powder for 10–1000 ms, so that the velocities of material displacement reach 10–100 m/s. These conditions, in combination with adiabatic heating, are favorable for effective overcoming of the strong particles coupling in nanopowders [8–10]. This provides compacted powder states of increased density, promotes destruction of particle agglomerates, and favors structural transformations resulting in microdistortions of the lattice, defects, and phase transitions. Thus the MPC seems to be a suitable method for pressure treatment of electrode coatings. Li-containing spinels are considered to be the promising candidates as electrode material [11]. Therefore in this work the influence of pulsed compaction treatment on characteristics of two types of well studied materials—LiMn2O4, Li4Ti5O12 [11–13] has been investigated. Compressibility, structure, adhesion strength and electrochemical characteristics of these electrode materials have been studied.
2780
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
2. Experiment Two types of electrodes based on the commercial nanopowders LiMn2O4 and Li4Ti5O12 (Honeywell) have been used in our experiments. The powders were mixed with graphite, carbon black and polyvinyl difluoride (PVdF) binder at ratio of 80:10:3:7 by weight. Then the solvent (NMP) was added in the mixture until the slurry was obtained. The slurry was spread on aluminium foil used as a substrate and then dried at the temperature of 140 °C during 1 h under vacuum. The tailored electrode coatings based on LiMn2O4 and Li4Ti5O12 powders are abbreviated below as LMO and LTO, accordingly. The compressibility of both electrode materials has been investigated using uniaxial MPC treatment. The pressure pulses were characterized by amplitudes of up to 0.7 GPa and of about 300 μs in duration. The disk-shaped samples 32 mm in diameter have been compacted between two polished steel disks. In order to investigate the influence of heat treatment on the compacts properties a series of samples was pressed at the same pressure of 0.45 GPa and then annealed at different temperatures up to 220 °C during 3 h. The densities of the coating materials were determined via the thickness measurements with standard micrometer before and after compaction. The phase composition and structure of the materials were analyzed by X-ray diffraction (XRD) method using a DRON-4 diffractometer with filtered CuKα radiation. The microstructure of electrode materials was observed by atomic force microscopy (device Solver 47p) in profile (HEIGHT) and phase contrast (MAC⁎Cos) modes. The tensile adhesion strength between electrode material and aluminium substrate was measured on disk-shaped samples 22 mm in diameter. In order to perform a test, a sample was glued between flat surfaces of two metal parts of the tear testing machine. Then these two parts were pulled in different directions until the coating was detached from the substrate. This process was completely force controlled. The preliminary experiments had shown that the strength of gluing layer was 20 ± 0.5 MPa allowing to carry out the measurements of mechanical characteristics for the coatings with σ < 20 MPa. In order to investigate the influence of MPC and temperature treatments on electrochemical characteristics of the electrode
Fig. 2. X-ray diffraction patterns of uncompacted and compacted LMO and LTO coatings. C—graphite, A—aluminium.
materials, the comparison of operation of coin-cells made with the usage of electrodes prepared by different regimes has been made. Thereat the tested electrodes were fabricated from both compacted and uncompacted electrode foils. The pure Li was used as the counter electrode. The electrolyte was a liquid (1 M LiPF6 in EC:DMC 1:1) dispersed in a polymer separator material (Solupor). Both foil electrodes were used to assemble the cylindrical shape cells which are perspective for the commercial production. The pile which consisted of the anode (LTO) foil, the separator (Solupor), the cathode (LMO) foil and the insulator (Kapton) foil was coiled and placed into aluminum tube. The electrolyte was the same solution as used in the coin-cells. Two types of so prepared cylindrical cells have been tested. The first one was radially compacted by the use of IAP Magnepress, and the second one was not compacted. All cells were charged and discharged at a constant current rate and cycled within a certain voltage range. All the measurements were controlled and recorded automatically by a MACCOR S4000 battery tester. 3. Results and discussion
Fig. 1. Compressibility of LMO and LTO coating under action of pulsed pressure.
The density data of compacted electrode coatings are presented in Fig. 1. The symbols denote values of experimental data averaged by 10 samples and solid lines are their spline approximations. The densities of the electrodes coatings are shown in relative units for better comparison of the materials compressibilities because they have different theoretical densities. The relative density γ is the relation of the measured coating density to the theoretical density of the same coating material. The theoretical densities of LMO and LTO coatings, that were calculated based on the densities of the material components and their proportion in the composition, were
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
3.54 g/cm3 and 3.02 g/cm3, accordingly. Maximum densities of LiMn2O4 and Li4Ti5O12 have been found by X-ray investigation and were 4.28 g/cm3 and 3.51 g/cm3, accordingly. PVDF density was 1.78 g/cm3 and the density of carbon black was equal to the density of graphite and was 2.27 g/cm3. It can be seen that compressibilities of the coatings are practically equal though experimental data for LMO coatings stably exceed LTO data. The highest densities of LMO and LTO coatings compacted with pressure ∼ 0.7 GPa were 96% and 91%, accordingly. The X-ray research showed (see Fig. 2), that in both electrode materials, LMO and LTO, there were no substantial structural changes upon pressing and annealing. Some graphite and metallic aluminum diffraction lines can be seen at the X-ray diagrams of compacted coatings along with the base phase
2781
diffraction lines of active material. The relative intensities of LiMn2O4 and Li4Ti5O12 diffraction lines of pressed coatings are similar to those observed in the starting materials within the error range for their determination. The analysis of integral diffraction line widths showed that at small angles they are almost the same, but the line broadening in the large angles area was observed. This effect was more severe as higher pulsed pressure was applied. This line broadening can not to be explained by the decreasing of the particle size. Basic reason of the line broadening is the distortion of the spinel crystalline lattice both due to microdistortions and to packing defects. Figs. 3 and 4 illustrate the comparison of AFM images of starting, compacted and additionally annealed LMO and LTO coatings, respectively, in phase contrast mode. For both starting
Fig. 3. AFM images (MAG⁎cos mode) of the external surface of LMO coatings: a) uncompacted; b) compacted (0.45 GPa); c) compacted (0.45 GPa) and annealed (180 °C); d) compacted (0.45 GPa) and annealed (220 °C).
2782
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
Fig. 4. AFM images (MAG⁎cos mode) of the external surface of LTO coatings: a) uncompacted; b) compacted (0.45 GPa); c) compacted (0.45 GPa) and annealed (180 °C); d) compacted (0.45 GPa) and annealed (220 °C).
materials the images (Figs. 3a and 4a) reveal the structures with mean particle size in the range of 200–1000 nm. From these images, one phase seems to be dominant, which is probably associated with polymer-covered particles of active materials. The images of compacted coatings (Figs. 3b and 4b) obtained at pulsed pressure of 0.45 GPa show very tight packing of crystals that corresponds to almost full dense material. The fine-grained structure with mean size well below 100 nm is seen. It is clear that the destruction of agglomerates during MPC treatment takes place. Probably, the starting active powders of LMO and LTO consist of sub-micron agglomerates, which are covered by a polymer in the starting coatings. The next feature is the presence of at least two phases on the surfaces of compacted coatings. Light and dark zones on these images denote various phases. Additional annealing of the compacted coating results in more homogeneous distribution of polymer additive (Figs. 3c
and 4c). Moreover the annealing at the highest temperature (220 °C) led to polymer removal from the external surfaces (domination of light zones on Figs. 3d and 4d). Materials that were treated under such regime are characterized with the finest surface structure. In our opinion, the polymer transforms to the liquid state and penetrates inside the pores of the coating. Pulsed compaction decreases the adhesion strength of LMO and LTO coatings with Al foil as it is demonstrated in Fig. 5. The adhesion strength decreases upon increasing of the compacting pressure. But the annealing of the coatings at temperatures up to 220 °C after pulsed compaction leads to the partial restoration of adhesion strength in comparison with the strength that was measured before compaction (Fig. 6). In the case of LMO this value was up to 8,4 MPa, and for LTO coatings—11 MPa that was close to the adhesion strength of the starting coatings.
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
2783
Fig. 5. Influence of compacting pressure on adhesion strength for LMO and LTO coatings.
Most likely the reason of the adhesion strength decreasing during compaction is the destruction of the polymer layer binding of the coatings with the substrate due to pressing of the active material particles into it. The restoration of the adhesion strength after annealing is connected with the polymer binder viscosity decreasing upon heating, which leads to its flow and reconnection of the powder particles with the substrate. Fig. 7 shows the most typical experimental cycling results. It is clearly seen that MPC treatment of LMO electrode does not result in a considerable change in capacity behavior, while in the case of LTO electrode, cycleability (slope angle of the curve) and stability (absence of peaks on the curve) of cell operation has been improved after pulsed compaction essentially. A dependence of the cell capacity from the annealing temperature was not found for both electrode materials. Thereat the cells capacity with electrodes that were treated under different regimes fluctuated within 7% and 13% in the case of LTO and LMO, accordingly.
Fig. 6. Influence of annealing temperature on adhesion strength for uncompacted and compacted (0.45 GPa) LMO and LTO coatings.
Fig. 7. Capacity vs. cycle number for cells with uncompacted and compacted tested electrodes.
In Fig. 8 typical curves depicting behavior of the cells in the “charge–discharge” cycle are presented. These data also demonstrate no influence of pressure treatment on LMO electrode cells operation, while the cells that were made on the basis of compacted LTO electrodes showed more stable operation in comparison with uncompacted ones. It can be seen
Fig. 8. Typical charge–discharge curves of the uncompacted and compacted (0.45 GPa) LMO (a) and LTO (b) electrode cells.
2784
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785
that gaps between “charge” and “discharge” shoulders greatly decrease after the electrodes were compacted. It shows qualitatively the decrease of the internal energy loss in the coin-cell as a result of the compaction. The numerical evaluation of internal energy loss for the 10th “charge–discharge” cycle has been calculated for the tested cells by the following formula: d¼
W0 −W 100 W0
where δ—loss factor, W0—energy introduced in the cell during the charge, W—energy returned by the cell during the discharge. In Fig. 9 the portion of the lost energy per cycle versus the annealing temperature is presented. In the case of the LMO electrodes the loss factor does not depend on a heat treatment and is approximately equal to 3%. The loss factor of the compacted LTO electrode cells is ∼ 7% that is much lower than the loss level, 22%, for the uncompacted ones. In all cases the temperature dependence of the loss factor is negligible. As it was shown above significant changes of the chemical composition and structure of the coatings under action of pulsed compaction did not take place. Therefore most likely the reason of the enhancement of electric characteristic of the coin-cells with LTO electrodes is the change of the electrical contacts between particles inside the coatings and also the improvement of contact between the active material and the substrate. As it can be seen from Fig. 3 and 4 Li4Ti5O12 powder is more agglomerated in comparison with LiMn2O4 powder, therefore it has less contact area. After compaction the particles size sufficiently decreased resulting in contact improvement. The decreasing of LiMn2O4 particle size is insignificant that is probably explained by the indifference of the LMO coating to pressure treatment. The pressure treatment reduced the adhesion strength between the electrode coating and the substrate. In experimental conditions it did not influence the electrical characteristics of cells. Nevertheless the given parameter is important for a long-term service of lithium batteries, and as was shown above, can be restored by the subsequent heat treatment.
Fig. 10. Capacity vs. cycle number for uncompacted and compacted cylindrical cells.
In Fig. 10 the cycleability tests for cylindrical cells assembled with the LMO electrode as cathode and LTO electrode as anode data are presented. It can be seen that the compacted cell is characterized by the higher starting capacity and much lower falling rate of capacity in comparison with uncompacted cell. It is possible to explain the improvement of the electrical characteristics of the cylindrical cells processed by radial pulsed pressure after assembly, both by the improvement of structure of the electrodes, and by the reduction of interelectrode gaps. 4. Conclusions 1. The electrode coatings based on LiMn2O4 (LMO) and Li4Ti5O12 (LTO) are characterized by good compressibility under pulsed compaction. The relative densities reached 96% and 91%, accordingly, upon pressure pulses with the amplitude of ∼ 0.7 GPa. 2. Under the pulsed compaction the agglomerates (0.2–1 mkm) of active material break into the particles with the size well below 100 nm. There were no essential changes in crystalline structure (XRD) of the electrode coating under action of pulsed pressures up to 0.7 GPa. 3. Pulsed compaction decreased the adhesion strength of LMO and LTO coatings with Al foil. Moreover the adhesion strength decreased with the increase of the compacting pressure. However the annealing of the samples at temperatures of up to 220 °C after pulsed compaction led to the partial restoration of adhesion strength. 4. Preliminary pulsed pressure treatment of LTO electrode improved the cycleability and stability of cells operation essentially. The same treatment did not influence the operation of LMO electrode cells. 5. The electrical characteristics of cylindrical cells have been significantly improved by radial pulsed pressure treatment. References
Fig. 9. Loss factor of the uncompacted and compacted (0.45 GPa) LMO and LTO electrode cells for 10th cycle.
[1] A.N. Jansen, A.J. Kahaian, K.D. Kepler, P.A. Nelson, K. Amine, D.W. Dees, D.R. Vissers, M.M. Thackeray, Journal of Power Sources 81–82 (1999) 902.
A.V. Nikonov et al. / Solid State Ionics 177 (2006) 2779–2785 [2] D. Aurbach, Journal of Power Sources 146 (2005) 71. [3] A. Odani, A. Nimberger, B. Markovsky, E. Sominski, E. Levi, V.G. Kumar, M. Motiei, A. Gedanken, P. Dan, D. Aurbach, Journal of Power Sources 119–121 (2003) 517. [4] J.T. Son, Electrochemistry Communications 6 (2004) 990. [5] J. Molenda, Solid State Ionics 176 (2005) 1687. [6] M.J.G. Jak, F.G.B. Ooms, E.M. Kelder, W.J. Legerstee, J. Schoonman, A. Weisenburger, Journal of Power Sources 80 (1999) 83. [7] V.A. Mironov, Magnetic Pulsed Compaction of Powders, Znanie, Moscow, 1980. [8] V.V. Ivanov, S.N. Paranin, A.N. Vihrev, A.A. Nozdrin, Materials Sciences Transactions 5 (1997) 49.
2785
[9] V.V. Ivanov, A.A. Nozdrin, S.N. Paranin, S.V. Zayats, Physics–Chemistry of Ultradispersed Systems, 2001, p. 229. [10] V.V. Ivanov, S.N. Paranin, S.V. Dobrov, A.V. Nikonov, V.R. Khrustov, Proceedings of Ninth Int. Conf. Megagauss Magnetic Field Generation and Related Topics, Moscow, St.-Petersburg, July 7–14 2002, p. 132. [11] M.M. Thackeray, Journal of the American Ceramic Society 82 (1999) 3347. [12] E.M. Kelder, F.J.B. Ooms, R. Perego, J. Schoonman, Journal of Power Sources 68 (1997) 590. [13] K.M. Colobow, J.R. Dahn, R.R. Haering, Journal of Power Sources 26 (1989) 397.